Chair base



FIG. 1 is a front perspective view of a chair base showing the new design;

FIG. 2 is a front elevation view thereof;

FIG. 3 is a left side elevation view thereof;

FIG. 4 is a right side elevation view thereof; and,

FIG. 5 is a top plan view thereof.

The broken lines immediately adjacent the shaded surfaces represent the bounds of the claimed design. All other broken lines depict environmental subject matter that forms no part of the claimed design. The broken lines and unshaded surfaces bounded by broken lines form no part of the claimed design. 

CLAIM The ornamental design for a chair base, as shown and described. 